Generally, in a patterning process at the time of manufacturing a semiconductor integrated circuit, it is necessary to form a circuit pattern on a wafer coated with a resist by the use of a photomask (also called a reticle) with the circuit pattern drawn thereon (which is called pattern exposure or pattern drawing), and a system therefor is called an exposure system or an exposure apparatus.
On the other hand, in order to fabricate a photomask, it is necessary to provide, on the surface of a quartz plate or the like which will serve as a substrate of the photomask, a chromium film or the like for shielding exposure light in a pattern corresponding to an intended circuit pattern. This chromium film is formed by pattern drawing and, as a general system for performing such pattern drawing, an electron-beam mask drawing system (hereinafter abbreviated as an EB drawing system) using an electron beam is widely used.
In recent years, for adaptation to highly integrated and miniaturized semiconductor integrated circuits, the prices of photomasks have been increasing. One of causes of this is that as long as several tens of hours to several hundreds of hours are required for drawing each photomask by an EB drawing system.
Apart from the EB drawing system, a system based on a technique that performs pattern drawing using laser light in the ultraviolet region, which is called a laser-beam drawing system, has also been commercialized as a drawing system for use in the manufacture of photomasks.
As a conventional example of such a system, it uses a reflector display element (a mirror device called micromirrors or the like) having a large number of micromirrors arranged in a two-dimensional array and performs pattern drawing on a photomask substrate by irradiating laser light in the ultraviolet region onto the reflector display element and controlling reflected light therefrom into a pattern. This laser-beam drawing system can collectively draw a portion of a circuit pattern and therefore has an advantage that the drawing speed is fast. This is shown, for example, in Non-Patent Document 1 or Patent Document 1.
According thereto, in a laser-beam drawing system using a mirror device, the mirror device uses about one million (about 500×about 2000) micromirrors and each micromirror has a size with each side being about 16 μm. This is reduction-projected onto a photomask substrate at 1/160 magnification through a reduction-projection optical system.
As a result, a pattern corresponding to each micromirror (which is called a spotlight) becomes a square with each side of 0.1 μm, i.e. 100 nm (actually, it becomes a scattered light close to a circle with about φ100 nm due to a light-source wavelength limit). However, the minimum size (called a minimum grid) in design when drawing a photomask is 1 to 4 nm, which is far smaller than the spotlight. Therefore, the resolution of pattern drawing is improved by changing an amount of light irradiated onto a pattern to be projected. For example, according to the foregoing document, the amount of light is changed in 64 levels (using intermediate amounts of light), thereby adapting to a minimum grid of 1.56 nm being 1/64 of 100 nm. In such a drawing method using gradation-controllable spotlights, it is necessary to prepare in advance patterns to be displayed on a mirror device and display the patterns at proper positions on a photomask. The patterns to be displayed on the mirror device, position information, and so on are called drawing data and generally generated from a desired pattern (design graphic data) by a conversion program.
In the drawing system using gradation-controllable spotlights in a two-dimensional array like the foregoing laser-beam drawing system, it has been difficult to generate drawing data from design graphic data. There are roughly two causes for this. One is that while the size of each spotlight is about φ100 nm, the pattern accuracy to be controlled requires a very small value of about 1 to 4 nm. The second is that since the size of the spotlight is smaller than the wavelength of a light source, the spotlight is in the form of a scattered light. For these causes, the amount of light of each spotlight should be adjusted while taking into account overlapping with the adjacent spotlights around it. Although the pattern can be drawn with the intended accuracy in principle, the drawing accuracy remains at about several tens to several hundreds of nm when drawing general design graphic data (drawing is enabled with the intended accuracy for a very limited, simple pattern). Therefore, a method of generating a drawing data has been required which can drawing the entire general design graphic data with the accuracy of about 1 to 4 nm currently required for the photomask fabrication.    Non-Patent Document 1: Proceedings of SPIE, Vol. 4186, p.p. 16-21    Patent Document 1: U.S. Pat. No. 6,428,940